1. Field of the Invention
This invention relates to memory devices and, more particularly, optical holographic memories and means by which such memories can be accessed.
2. Description of the Prior Art
Optical holography shows great promise as a technique for storing and retrieving information. This is partly because it makes efficient use of the large storage capacity of modern photosensitive recording materials. In addition, a hologram has a number of properties not possessed by a conventional photograph.
A hologram is relatively insensitive to damage. Fingerprints, scratches, dust or breaks do not obscure the image formed from a hologram as they would obscure parts of a photographic image. Thus, important information contained in the image is not lost by damage to the hologram.
It is also possible, by multiply exposing the hologram, to store, in the same piece of photosensitive material, images of several different objects, recorded at different times. These images can be retrieved, one at a time, with relatively little interference from the other images of the multiple exposure. The images of the different objects are said, therefore, to be multiplexed in the hologram. The same cannot be said for conventional photography.
Finally, by holographic techniques, full use can be made of the storage capacity of a photosensitive material regardless of the volumetric configuration of that material. Since a thick photomaterial is capable of storing far more data than a thin material, this capability would be of great advantage in a memory device.
Unfortunately, existing methods of implementing a holographic data bank are clumsy. To see why this is so, a brief description of the typical holographic apparatus is necessary.
An optical hologram is a record of the interference pattern produced when two wavefronts of light overlap. The wavefronts must be mutually coherent so that the interference pattern is stationary and will not be smeared during the recording process. Mutual coherence merely means that there is a fixed phase relationship between the spatially overlapping parts of the two wavefronts.
In normal practice, the two wavefronts are derived from a single, sufficiently coherent light source by means of a beam splitting device. A half silvered mirror in the beam of a laser is often suitable for the production of the two wavefronts.
One of the two wavefronts is scattered from some object, thenceforth carrying with it information about the visual appearance of the object. In the context of information storage, the object from which this wavefront is scattered is called a "page composer." The page composer impresses on the wavefront a stylized pattern which may be interpreted by some conventional readout device. The page pattern may be altered in accordance with the information being transmitted.
The information about the appearance of the object or page composer is located primarily in the spatial pattern of phase relationships contained in the scattered object (or signal carrying) wavefront. The hologram is capable of storing this phase pattern. This is possible because a second, or reference, wavefront, when superimposed on the object wavefront, acts as a phase reference standard. The two wavefronts interfere and the interference shows up as a pattern of light and dark. This interference pattern can be recorded by placing a piece of photosensitive material in the region of space where the two wavefronts overlap and interfere. This recording is the hologram.
During the reconstruction process, the true object wavefront is absent, either because the scattering object has been removed or because this wavefront has been blocked. The reference wave passes through the developed hologram as it did during the recording period, but now it is spatially modulated by the pattern recorded there. The hologram impresses again on the wavefront the interference pattern which once existed. When this pattern has been impressed, the wavefront is changed into a replica of the two wavefronts which originally produced the interference. Hence, the wavefront will propagate "downstream" of the hologram as if the original object wave, as well as the reference, were still present. The object wave is therefore said to have been reconstructed.
Traditionally, in order to maintain a constant intensity reference field across the hologram, the reference wave has been formed as a plane or spherical wave. Holographic data storage devices which use this type of reference wave necessarily must be clumsy in construction and, usually, slow in operation.
The reason is that plane or spherical waves are not really suited to serve as reference waves for a multiplexed hologram. If the photomaterial is in the form of a thin sheet, the images preserved in a multiply exposed hologram, referenced by plane waves, will have a tendency to overlap and conflict. To circumvent this problem, the holograms of separate objects are not multiplexed into the same region of the recorder. Rather, they are laid side by side on the sheet of photomaterial in the form of an album of nonoverlapping holograms. While this arrangement avoids the problem of conflict during the reconstriction, it does require that both the reference and object beams be deflected to different positions. This requirement means that the holographic apparatus takes up excessive space and requires one or more beam deflectors.
A light beam cannot be easily deflected from its path by electromagnetic means. Thus, optical beam deflectors usually are partly mechanical in nature. A motor rotated mirror or an acoustic Bragg diffraction cell are two examples of popular beam deflectors. These devices suffer from a number of undesirable characteristics, but lack a flexibility, lack of speed, large size and high power consumption are dominant.
If a thick photosensitive material is used to record the holograms, true multiplexing may be obtained even when plane waves are used as references. This is because the interference patterns in a thick hologram form Bragg diffraction surfaces which are spatially selective in nature. By somewhat changing the angle of incidence of the plane or spherical wave used to reference each of the multiplexed holograms, the holograms may be kept separate. Each hologram may be individually reconstructed by reintroducing the reference wave at precisely the same angle of incidence that was used when that hologram trace was formed. Unfortunately, this format for a holographic apparatus also requires optical beam deflectors for control of the reference wave.
The thick hologram is also color selective so that multiplexing may be accomplished by using a different wavelength of light for each exposure. This is not at present a practical technique since rapidly tunable light sources of sufficient coherence and brightness are not as yet available.
There is another type of wavefront, radially different from a simple plane or spherical wave, which may be used successfully as a reference wave. This is a wavefront which, at a distance far enough from the hologram region to be considered the far field, possesses a very complicated phase and amplitude pattern. This pattern must possess an autocorrelation function which is much narrower than any distinguishable part of the pattern of information impressed by the page composer on the object wavefront.
A random pattern is often capable of meeting these criteria. When these conditions are met, the reference wave will behave during the reconstruction as if it were of constant intensity over the region of space spanned by the hologram. A hologram which uses a complicated wavefront of this type as a reference wave is denoted a "ghost image," or sometimes a "source compensation," hologram.